pregnancy after a tubal ligation?

How likely is a pregnancy after a tubal ligation? I'm 8 months postpartum, and missed my period. I've always been on time with my period especially after having the baby. The past 3 days I've had nausea and today I'm having hot flashes. My fiancé keeps telling me I'm pregnant, but I keep telling him it's not possible since I had my tubal ligation Done. (Mine are just bands, or clips the doctor didn't specify which ones he used.)